SEDIMENT STORAGE ON A 1ST ORDER STREAM FLOOD PLAIN USING GPR

Sediment storage and deposition on 1^st and 2^nd order stream channels has been a growing area of interest in the mid-Atlantic due to sediment and nutrient loading challenges in the Chesapeake Bay. Our study involves a small 1^st order gravel bedded stream surrounded by farms with an easily identifiable flood plain. Using hand dug test pits and a 100mhz GPR, we found there is a relatively uniform gravel layer across the entire flood plain. We interpret this layer to represent conditions following the last glacial maximum. This layer suggests a broad multi channelized stream system which is a distinct contrast to the present1^st order, incised meander system. We used ArcMap to digitize the study area and calculate both the area of the flood plain and the surrounding farm fields. This allowed us to determine the volume of sediment stored and provided a rough estimate of the soil denundation across the adjacent farm field. The total volume of the clay and silt sized sediment stored on top of the gravel layer in our study area is 1414 m³ which corresponds to a vertical loss from the adjacent farm field of 6.4cm. Field edges that are believed to represent pre-agricultural surfaces support erosional loss of up to 1m of topsoil in the surrounding catchment. Our results indicate that the amount of sediment stored on the flood plain is significantly lower than other 1^st order stream systems and that the majority of the sediment eroded off the farm fields is transported downstream.